1. Field of the Invention
The present invention relates to an optical balanced receiver and an IQ receiver with balanced compensation, and particularly to an optical balanced receiver and an IQ receiver with balanced compensation for compensating or correcting distortion of waveforms of received signals due to degradation in the output balance between two photodetectors of an optical balanced receiver in optical information transmission using optical fibers.
2. Description of the Related Art
Optical multilevel modulation which is being promoted to be practically used from the viewpoint of the frequency efficiency is improved in receiver sensitivity by using an optical balanced receiver for optical phase detection.
FIG. 1 shows an example of the construction of an optical balanced receiver. The optical balanced receiver 100 comprises an optical coupler 101, two photodetectors (O/E) 102, 103 and a difference device 104. The optical coupler 101 combines waves of an optical information signal 201 and an optical reference signal 202 and outputs two optical interfering signals 203 and 204 whose phases are opposite to each other. The optical interfering signals 203 and 204 are received by photodetectors 102 and 103 respectively and converted to detection signals 205 and 206 having the amplitudes corresponding to the interference intensities thereof. The difference device 104 obtains the difference between the two detection signals 205 and 206 and outputs it as a received signal 207.
FIG. 2 shows an example of the construction of using this optical balanced receiver 100 for differential phase detection. In this construction, the optical coupler 101 shown in FIG. 1 is substituted by an optical delayed interferometer 301 having an optical splitter 302 and the optical coupler 101. In the optical delayed interferometer 301, the input optical information signal 201 is split into an optical information signal 305 and an optical reference signal 306 by the optical splitter 302, and these two signals are subjected to a differential delay of an integral multiple of a signal period and any phase shift by using an optical delay line 303 and an optical phase shifter 304. Thereafter, these signals are combined with each other in the optical coupler 101. The optical delay line 303 and the optical phase shifter 304 may be disposed in any one of respective transmission paths for the optical information signal 305 and the optical reference signal 306.
In this optical balanced receiver 100, two paths passing through the optical detectors 102 and 103 respectively and extending from the output terminal of the optical coupler 101 to the difference device 104 are balanced to each other in loss and delay amount (hereinafter collectively referred to as “receiver balance”), whereby the common components of the two optical interfering signals 203 and 204 are offset by each other and an intensity fluctuation component contained in the optical information signal 201 or the optical reference signal 202 is removed from the received signal 207. The receiver sensitivity and the delay amount of the two photodetectors 102 and 103 are also contained in the receiver balance.
For example, the relation between the receiver balance and the degradation of the receiver sensitivity when the optical information signal 201 is subjected to Quadrature Phase Shift Keying is disclosed in FIG. 9 of “Phase Diversity Receivers for Homodyne Detection of Optical DQPSK Signals”, Journal of Lightwave Technology, vol. 24, No. 9, September 2006 (hereinafter referred to “non-patent document 1”). In the case of DQPSK (Differential Quadrature Phase Shift Keying), the receiver sensitivity is degraded by about 1 dB when the sensitivity ratio of photodetectors is equal to 10% or the difference in path length of the photodetectors is equal to several millimeters. This degradation is more remarkable in optical multilevel modulation in which the multilevel number is increased, such as optical 8-phase shift keying, quadrature amplitude modulation or the like.
Furthermore, there are disclosed a method of inserting an attenuator at the output side of the optical coupler or the photodetector so that the receiver balance can be adjusted (for example, see JP-A-2006-217605; hereinafter referred to as “patent document 1”), and an optical balanced receiver constructed by an offset-compensation affixed difference device and an offset control circuit (for example, see “Balance type Photodiode Module Technique”, NTT Technology Journal 2007. 11, pp 58-61; hereinafter referred to as “non-patent document 2”).